The subject matter of this invention concerns a process for capturing arsenic using a mass of lead deposited on alumina, wherein the active phase is in the oxide form regarding a portion of the catalytic bed, preferably the major portion of the catalytic bed, and in the pre-sulphurised form regarding the other portion. Mercaptans are known to be powerful arsenic capture inhibitors. However, it has been observed that pre-sulphurising a portion of the catalytic bed can result in a very good arsenic capture gradient over the remainder of the bed even when mercaptans are present in the feed, in contrast to that observed in the absence of a pre-sulphurisation step.
Processes for cracking heavy petroleum cuts, for example catalytic cracking, visbreaking or cokefaction, produce light cuts that are strongly contaminated with various compounds containing sulphur, nitrogen and oxygen. Arsenic is often detected alongside those impurities.
The sulphur-containing compounds are usually hydrogen sulfide and mercaptans. The nitrogen-containing compounds, present in the light cuts, are principally ammonia or light amines. Arsenic is itself also present in the form of compounds with the general formula AsR3, R being a hydrocarbon radical such as CH3 or a hydrogen atom.
The term xe2x80x9clight cutsxe2x80x9d as used here means those that are gaseous under normal pressure and temperature conditions, i.e., C2, C3 or C4 cuts. Such cuts are generally treated to eliminate sulphur-containing compounds. In particular, C3 and C4 cuts usually undergo an amine washing treatment, followed by washing with sodium hydroxide. Those different washes eliminate almost all of the H2S, only a portion of organic sulphur-containing compounds such as mercaptans, and only extract COS in a very incomplete fashion.
In general, C3 and C4 cuts, which contains a large proportion of olefins, constitute a high value starting material for the production of fuels or chemical products such as certain polymers. These transformations involve a variety of catalytic treatments in which the catalysts are poisoned at varying rates by sulphur-containing or arsenic-containing compounds.
Recently, a process for capturing the arsenic contained in hydrocarbons in the gas phase (U.S. Pat. No. 3,782,076) or in the liquid phase (U.S. Pat. No. 4,849,577) have been described, carried out at relatively high pressures (more than 2 MPa) in the presence of sulphur-containing compounds. That process uses an absorbent mass comprising lead oxide and a xe2x80x9cnon acidicxe2x80x9d support, i.e., it does not catalyse reactions known to the skilled person to be catalysed by acidic solids, namely hydrocarbon skeletal isomerisation, cracking and polymerisation.
Following the washing treatments cited above, the C3 cut from fluidised bed catalytic cracking (FCC) still contains COS and/or mercaptans in amounts of the order of 1 to 50 ppm by weight, and arsenic in amounts of the order of 0.1 to 5 ppm by weight.
Thus, providing an arsenic capture mass that can decontaminate a feed even when said feed contains compounds such as mercaptans which may contaminate that mass, would be desirable.
This capture mass is employed in any method that can bring the fluid to be decontaminated into contact with the lead mass.
The invention concerns a process for eliminating arsenic from a hydrocarbon cut, in which said cut is brought into contact with an absorption mass that is at least partially pre-sulphurised and comprises a support and lead oxide.
The support for said mass preferably has a specific surface area in the range 10 to 300 m2/g, a total pore volume in the range 0.2 to 1.2 cm3/g and a macroporous volume in the range 0.1 to 0.5 cm3/g. The lead content of said mass, expressed as lead oxide, is preferably in the range 5% to 50% by weight. The fraction of the sulphurised mass preferably represents at least {fraction (1/20)}th of the total volume of the absorption mass.
The present invention concerns a process for eliminating arsenic from a hydrocarbon cut in which said cut is brought into contact with an absorption mass that is at least partially pre-sulphurised and comprises a support and lead oxide.
The support used in the present invention can be any support that is known to the skilled person. As an example, and preferably, alumina, silica or magnesia is used, more preferably alumina, which can produce both relatively large specific surface areas and sufficient mechanical strength.
The recommended support for the invention is an alumina with a surface area in the range 10 to 300 m2/g, preferably in the range 50 to 200 m2/g. Its total pore volume is preferably in the range 0.2 to 1.2 cm3/g, more preferably in the range 0.5 to 1.2 cm3/g. The macroporous volume, defined as that corresponding to pores over 100 nm, is preferably in the range 0.1 to 0.5 cm3/g, more preferably in the range 0.15 to 0.45 cm3/g.
The absorbent mass containing lead oxide can be prepared using any technique that is known to the skilled person. It is prepared by mixing a lead compound with the support, using known techniques. One preparation procedure that routinely leads to a high performance mass is xe2x80x9cdryxe2x80x9d impregnation, i.e., filling the pores of the support with an aqueous solution of a lead salt by a volume equal to the pore volume of the support. Any sufficiently soluble lead salt can be used, such as lead nitrate or lead acetate. Preferably, lead acetate is used, as it has a satisfactory solubility and can produce a capture mass with a high efficiency.
After impregnating the support with the solution of the lead compound, the mass is heated to a temperature in the range 300xc2x0 C. to 700xc2x0 C., preferably in the range 400xc2x0 C. to 550xc2x0 C., to convert the lead compound into lead oxide. Preferably, an atmosphere containing oxygen is employed.
The masses obtained advantageously comprise 5% to 50% by weight of lead, preferably 10% to 45% by weight, more preferably 15% to 40% by weight of lead, these percentages being expressed as lead oxide.
Absorption is carried out at a temperature that is preferably in the range 5xc2x0 C. to 150xc2x0 C., ore preferably in the range 10xc2x0 C. to 100xc2x0 C., at a pressure that can maintain the cut to be treated either in the gas phase or in the liquid phase, for example, in the range 0.1 MPa to 4 MPa, preferably in the range 0.5 MPa to 2.5 MPa. The absorbent mass is thus in different chemical forms: an oxide form and a pre-sulphurised form.
When the amount of sulphur-containing contaminants (COS, H2S, mercaptans) is low, a capture mass is preferably used in the process of the invention that preferably comprises only a very small proportion of pre-sulphurised mass, for example {fraction (1/20)}th to {fraction (1/10)}th of the total volume of the absorption bed or beds, more preferably {fraction (1/20)}th to {fraction (1/15)}th of the total volume of the absorption bed or beds.
When the feed to be treated has a higher sulphur-containing contaminant content, more particularly mercaptans, then preferably the capture mass used is in an at least partially pre-sulphurised form, preferably in a proportion of at least {fraction (1/15)}th of the total volume of the catalytic bed, more preferably at least {fraction (1/10)}th and very preferably at least ⅕th.
It has been shown that the absorbent mass has a poor arsine capture gradient when the feed to be decontaminated has a non negligible mercaptan content. Mercaptans usually cause severe inhibition of arsenic capture.
In contrast, pre-sulphurising the capture mass can produce a very good capture gradient in the remainder of the catalytic bed, which remains in the form of lead oxide PbO, deposited on the support.
This first layer of pre-sulphurized absorbent can decompose mercaptans into sulphur-containing compounds that are not poisons and do not impair arsenic capture over the remainder of the catalytic bed, which remains in the oxide form.
Pre-sulphurisation of the capture mass is preferably carried out with any sulphur-containing compound with the exception of mercaptans. Examples that can be cited are COS or hydrogen sulphide, alone or as a mixture with an inert gas or hydrogen.
This pre-sulphurisation can be carried out in situ, i.e., in the absorption reactor, or ex situ, i.e., outside the absorption reactor and preferably offsite, i.e., generally by an enterprise accustomed to offsite catalyst sulphurisation.
When the treatment is carried out in situ, the fraction of catalytic bed to be pre-sulphurised is preferably isolated, for example by optionally disposing the absorption mass in a plurality of successive beds inside the same reactor, or by placing the fraction of the absorption mass to be pre-sulphurised in a separate reactor, preferably located upstream of the other reactor or reactors.
Preferably, the sulphurised fraction and the oxide fraction of said mass are distributed in at least two reactors disposed in series. More preferably, the sulphurised fraction of said mass is disposed in a separate reactor located upstream of at least one other reactor containing the fraction of said mass in its oxide form. However, it is possible to place the pre-sulphurised fraction and the oxide fraction of said mass in a single reactor.
The following examples illustrate the present invention.